1. Field of the Invention
The present invention relates to an optical member and a method for producing the same.
2. Description of the Related Art
In general, individual lenses, which constitute an optical system of, for example, a camera lens and an objective lens of microscope, each have a surface which is coated with an anti-reflective film (optical thin film) so as to reduce the reflection of the light. A lens, on which such an optical thin film is formed, is disclosed, for example, in Japanese Patent Application Laid-open No. 2005-148551.
In general, the optical thin film as described above has been hitherto formed by the vapor deposition method including the sputtering method, the vacuum vapor deposition method, etc. According to the method for forming the optical thin film as described above, it is possible to form the optical thin film which has high performance and which realizes the low reflection in a wide wavelength range. However, in the method for forming the optical thin film as described above, it is difficult to form an optical thin film, having a uniform thickness (film thickness), on a surface of a lens having an intense curvature; a problem arises such that the film is thinned especially at circumferential or peripheral portions of the lens. Therefore, in the optical member obtained by adopting the method for forming the optical thin film as described above, the reflectance greatly differs (or varies) between the central portion and the peripheral portion of the lens in some cases, resulting in a problem such that the imaging performance is lowered, and/or ghost or any flare appears, etc.
In view of the above, in order to fundamentally solve the problems as described above, the following method has been adopted as a method for producing an optical member. That is, a wet film formation method, which is excellent in the performance of uniform film formation, is adopted to form an optical thin film on a surface of a lens (lens surface). In such a wet film formation method, the optical thin film can be formed by utilizing such a property that the coating liquid or solution flows following the curved surface of the lens. Therefore, an optical thin film, which has a uniform thickness, can be formed relatively easily. Among examples of the wet film formation method as described above, the spin coating method (spin coating) is a method capable of forming a film having a uniform film thickness as compared with the vapor deposition method, because the film is formed such that a coating liquid, which is applied to or coated on a lens, is spread thinly and uniformly by the centrifugal force brought about by rotation.
However, the conventional method for producing the optical member, which adopts the spin coating as described above, is not necessarily sufficient in relation to the formation of an optical thin film having sufficiently high uniformity of the film thickness. For example, in a case of an optical thin film is formed on a lens in which the ratio (D/R) between the diameter (D) of the lens and the radius of curvature (R) is close to 2, the coating liquid is moved, during a period until the coating liquid is dried, toward a peripheral portion of a lens at which the centrifugal force is applied more strongly, which in turn results in the coating liquid being pooled in the peripheral portion. This causes a problem such that the film thickness is thickened or great at the peripheral portion. In a case that an optical thin film is formed on a lens which has a relatively small diameter and which is to be used for an objective lens of a microscope, the following tendency arises. That is, the centrifugal force is hardly applied even when the rotation is performed at a high speed; and thus the coating liquid is inadequately separated and discontinued, and a thick liquid pool appears at the peripheral portion of the lens.
Further, in the above-described conventional method for producing the optical member in which the spin coating is adopted, when the lens is detached from a spin coater, the coating liquid is not cured on the coat surface (coated surface) immediately after the coating was performed. Therefore, it is necessary to take precautions not to touch the coat surface, wherein the operability is not sufficient. In particular, in a case that the lens is small and/or that the edge thickness is small, then there is a high possibility that the coat surface is touched, thereby further lowering the operability. Further, in a case that the spin coating is adopted, the coating liquid coated on a surface of the lens flows and arrives at the other surface (back surface) on the side opposite to the coat surface, depending on the shape of the lens. In particular, when the lens is chucked by the sucking, the following problem arises. That is, the coating liquid is sucked through the gap between the lens and a lens-fixing jig, and thus the back surface is dirtied by the sucked coating liquid.